This invention relates to dowel systems which can be employed as tying members between concrete bodies, and more particularly, to dowel systems which facilitate load transfer and dowel slippage across slab joints so as for maintaining the structural integrity of concrete slabs.
Concrete responds to changes in temperature and moisture when movement associated with these changes (or for other reasons such as internal chemical reaction) is restrained. In these instances stresses develop that can lead to cracking. To control cracking, joints are built at interval distances short enough to maintain stresses in the concrete below certain critical values. Transverse joints are saw cut, placed through induced cracking, or formed at predetermined spacings.
Concrete pavements for highways, airport runways and the like are generally placed in strips or lanes with a longitudinal joint formed between adjacent strips or lanes. Concrete is poured in the first strip and allowed to cure. Subsequently, concrete is poured and cured in the adjacent strip and so on until the concrete pavement is completed. A longitudinal joint is formed between adjacent strips to facilitate construction and to reduce stresses and control cracking caused by contraction or expansion of the concrete. Transverse or slug joints are also formed in concrete by cutting or sawing the concrete at a given location and to a given depth.
Similarly, joints are formed in concrete structural slabs, walls, footings and the like to minimize stresses and/or simplify construction methods. Of these joints, there are several types. For example, the expansion joint provides a space between slabs to allow for expansion or swelling of the slab as temperature and moisture increase or growth due to any cause occurs. A construction joint provides a finished edge or end so that construction operations interrupted for some length of time may be continued or resumed without serious structural penalty.
Load is transferred across a joint principally by shear. Some bending moment may be transferred across the joints through tie joints. Good load transfer capability must be built into the joint, or the load carrying ability of the concrete slab or structure will be reduced. The alternative is to strengthen the concrete by improving support or increasing depth to minimize the joint load transfer weakness.
Tie bars and dowels are often used in concrete design to improve load transfer at the joint between concrete bodies such as slabs or structures. Such tie bars and dowels are embedded in the concrete and arranged across the joint in a direction substantially perpendicular to the axis defined by the joint. Various approaches, depending on the type of tie bar or dowel, have been suggested with respect to concrete construction joints.
In the construction of concrete slabs on grade, it is common practice to install continuous side forms with dowels for future adjacent slab concrete placement and to place concrete in long continuous strips. It is also known to place slab dowels and sleeves at specified distances across the strips to allow the strips to have a controlled plane to accommodate shrinkage of the concrete. The positions of these dowel locations are marked on the side forms and the concrete after placement and finishing is struck to provide a joint at these locations, or is later sawn. This allows for a smooth controlled joint across the slab strip. However, many times the marks are destroyed and joints are placed in the wrong areas negating the advantages of the slab dowels.
The functions of the tie bars and dowels are to keep contiguous sections of concrete in alignment during contraction and expansion, and to transfer shear stresses and bending moments across the joint between adjacent slabs. The prior art dowels are often made smooth, lubricated, or coated entirely with plastic as disclosed in U.S. Pat. No. 3,397,626 to prevent the dowel from bonding to the concrete and allow the concrete slab or structure to slide relative to the dowel in a direction substantially perpendicular to the axis defined by the joint. Such movement of the slab relative to the dowel prevents build up of stress in the dowel that may result in cracking of the concrete.
In an alternative construction disclosed in U.S. Pat. No. 4,449,844, the dowel has its outer ends bonded to concrete and its central portion covered with plastic to prevent bonding to concrete. The dowel disclosed performs a latent spring function to limit the movement of the concrete slab relative to the dowel when temperature changes cause the length of the slab section to vary with time.
A major disadvantage of the above prior art dowels and tie bars is that they prevent movement of the concrete slab relative to an adjacent concrete slab in a direction substantially parallel to and aligned with the axis defined by the joint. In such situations, the dowels and tie bars provide enough restraint against movement and shrinkage so that the concrete slab or structure induces stresses along a line substantially defined by ends of the dowels or tie bars. This problem is most evident in the situation where adjacent concrete slabs or strips are placed and cured in repetitive order, or when adjacent concrete slabs or structures are subjected to extreme temperature differences.
For example, it is well known that concrete typically shrinks after formation. If a second concrete paving slab is placed adjacent to a first concrete paving slab that has contracted from thermal and drying shrinkage, the second concrete paving slab will likewise attempt to shrink in a manner similar to the shrinkage of the first concrete paving slab. However, dowels and tie bars arranged across the joint between the first and second concrete paving slabs will restrain the second concrete paving slab from shrinking during curing. The developed internal stress in the second concrete paving slab can create an undesirable condition that may result in cracking. Even if cracks do not develop, the internal stresses are added to the stress from the normally applied design loads and could reduce the service life of the pavement.
Another prior art slab dowel system, U.S. Pat. No. 4,578,916, relates to a connecting and pressure-distributing element for two structural members to be concreted one after the other in the same plane and separated by a joint, of the type having a socket and a bar insertable into the opening of the socket. The socket is inserted for attachment to a frontal concrete form and for embedding in the structural member to be concreted first. The bar is inserted into the socket hole and is intended for embedding in the structural member to be concreted later. The bar has at least two closed loops each of generally rectangular shape and made from reinforcing rods. The loops are secured to the socket and the bar, respectively, in one case by welding, in another case by means of a holder, because they are symmetrically spaced from the socket and the bar, they ensure good distribution of pressure within the concrete.
An improved tying bar and joint construction for transferring stresses across a joint between concrete slabs or structures and accommodating for shrinkage and expansion of concrete is provided in U.S. Pat. No. 4,733,513. The subject bar has a resilient facing attached to at least one side of the bar so that the concrete slab or structure can move in relationship to the bar in a direction substantially perpendicular to the resilient facing. The bar is arranged across the joint in a direction substantially perpendicular to the axis defined by the joint.
In U.S. Pat. No. 5,005,331, slip and non-slip dowel placement sleeves are disclosed. The slip dowel placement sleeve generally comprises a tubular dowel receiving sheath having a closed distal end and an open proximal end. A connecting means of perpendicular flange is formed around the proximal opening of the sheath to facilitate attachment of the sheath to a concrete form. Smooth sections of dowel rod may then be advanced through holes drilled in the concrete form and into the interior compartment of the sheath. Concrete is poured within the form and the dowel rod remains slidably disposed within the interior of the sheath. Variations of the basic slip dowel placement sleeve of the invention includes a tapered “extractable” sleeve and a corrugated “grout tube” for placement of non-slip dowel or rebar.
Slip and non-slip dowel placement sleeves are disclosed in U.S. Pat. No. 5,216,862. The slip dowel placement sleeve generally comprises a tubular dowel receiving sheath having a closed distal end and open proximal end. A connecting means is formed around or inserted into the proximal opening of the sheath to facilitate attachment of the sheath to a concrete form. Smooth sections of dowel rod may then be advanced through holes drilled in the concrete form and into the interior compartment of the sheath. Concrete is poured within the form and the dowel rod remains slidably disposed with the interior of the sheath. Variations of the basic slip dowel placement sleeve of the invention include a tapered extractable sleeve and a corrugated grout tube for placement of non-slip dowel or rebar.
In U.S. Pat. No. 5,713,174 and U.S. Pat. No. 5,797,231, a concrete dowel slab joint system, including a collapsible spacer member, is provided.
All of the U.S. patents cited above are incorporated herein in their entirety by reference.